The United States Bureau of Standards states that oil does not wear out mechanically. However, oil does get contaminated by exposure to an engine's environment. These contaminants are primarily soot, dirt, sludge, water, fuel metals, acids and oxidation products which gradually degrade the oil and are factors that ultimately break down the oil to reduce its effectiveness and contribute to engine wear.
There are two main contributing factors that are responsible for the breakdown of the integrity of the lubricating oil and the subsequent engine wear and ultimate damage: The first is particulate or mechanical, consisting of dust or grit from the air intake, carbon from combustion and wear metals from the metal to metal contact in the engine. The second factor is chemical in origin. Exposure to the high temperature of engine operation along with the combustion gases, such as CO, CO.sub.2, and SO.sub.2, combine with the moisture present in the lubricating system to form various acid based compounds harmful to both the oil and the engine.